The following relates to the illumination arts, lighting arts, and related arts.
Solid state lighting devices include light emitting diodes (LEDs), organic light emitting diodes (OLEDs), semiconductor laser diodes, or so forth. While adjustable color solid state lighting devices are illustrated as examples herein, the adjustable color control techniques and apparatuses disclosed herein are readily applied to other types of multicolor light sources, such as incandescent light sources (for example, incandescent Christmas tree lights), incandescent, halogen, or other spotlight sources (for example, stage lights in which selectively applied spotlights illuminate a stage), or so forth.
In solid state lighting devices including a plurality of LEDs of different colors, control of both intensity and color is commonly achieved using pulse width modulation (PWM). For example, Chliwnyj et al., U.S. Pat. No. 5,924,784 discloses independent microprocessor-based PWM control of two or more different light emitting diode sources of different colors to generate light simulating a flame. Such PWM control is well known, and indeed commercial PWM controllers have long been available specifically for driving LEDs. See, e.g., Motorola Semiconductor Technical Data Sheet for MC68HC05D9 8-bit microcomputer with PWM outputs and LED drive (Motorola Ltd., 1990). In PWM, a train of pulses is applied at a fixed frequency, and the pulse width (that is, the time duration of the pulse) is modulated to control the time-integrated power applied to the light emitting diode. Accordingly, the time-integrated applied power is directly proportional to the pulse width, which can range between 0% duty cycle (no power applied) to 100% duty cycle (power applied during the entire period).
Existing PWM illumination control has certain disadvantages. They introduce a highly non-uniform load on the power supply. For example, if the illumination source includes red, blue, and green illumination channels and driving all three channels simultaneously consumes 100% power, then at any given time the power output may be 0%, 33%, 66%, or 100%, and the power output may cycle between two, three, or all four of these levels during each pulse width modulation period. Such power cycling is stressful for the power supply, and dictates using a power supply with switching speeds fast enough to accommodate the rapid power cycling. Additionally, the power supply must be large enough to supply the full 100% power, even though that amount of power is consumed only part of the time.
Power variations during PWM may be avoided by diverting current of each “off” channel through a “dummy load” resistor. However, the diverted current does not contribute to light output and hence introduces substantial power inefficiency.
Existing PWM control systems are also problematic as relating to feedback control. To provide feedback control of a color-adjustable illumination source employing existing PWM techniques, the power level of each of the red, green, and blue channels must be independently measured. This typically dictates the use of three different light sensors each having a narrow spectral receive window centered at the respective red, green, and blue wavelengths. If further division of the spectrum is desired, then the problem then becomes very expensive to solve. If for instance a five channel system has two colors that are very close to one another, only a very narrow band detector is able to detect variations between the two sources.